Our laboratory is studying the bioactivity of leptin, a novel protein that regulates the size of the adipose tissue mass (see highlight #3, attached paper, and references therein). The efforts of the Laboratory of Mass Spectrometry has been of critical importance in these studies. Firstly, mass spectrometry have been used to confirm that recombinant leptin expressed in bacteria and yeast is of the correct molecular mass. This information was of great importance in confirming that the correct protein had been synthesized prior to assays of in vivo bioactivity. Secondly, mass spectrometry is being used to determine the molecular mass of the native protein in mouse and human plasma. This will have important implications for efforts to synthesize a recombinant protein with optimal potency. Proteolysis experiments using MALDI-MS are being used to probe for the flexible and accessible regions of the protein and to assess the lifetime of the protein in-vivo. The recombinant form of the protein appears highly susceptible to adventitious proteolytic degradation.